Steam Etching Research Articles

Synthesis of hierarchically porous Cu-BTC through phase-controlled etching

To enhance diffusion channels and boost active sites within microporous metal–organic frameworks, hierarchically porous structures are indispensable. This study pioneered phase-controlled acetic acid etching techniques, encompassing solution etching and steam etching resulting in the successful synthesis of a series of hierarchically porous Cu-BTC (HP-Cu-BTC). In contrast to etching with an acetic acid solution, acetic acid steam etching offers advantages in terms of reduced etchant usage and minimized damage to the original microporous structure. Noteworthy, by employing steam-controlled recovery etching technology, it is possible to regulate both the steam concentration and the etching degree, while also facilitating the recovery of etchant. The prepared HP-Cu-BTC exhibits adjustable pore sizes (15–45 nm), and mesopore volume (0.14–0.23 cm3/g). Compared to the original Cu-BTC, the well-engineered HP-Cu-BTC demonstrates an 81 % increase in ethylene diffusion rates. The phase-controlled synthesis of HP-MOFs serves as a valuable source of inspiration for the future preparation of hierarchical porous structures.

Synthesis of Carbon Nitride Nanosheets with n→π* Electronic Transition for Boosting Photocatalytic CO2 Reduction

Awakening n→π* electronic transition in graphitized carbon nitride can extend the visible light absorption range of the original g-C3N4, which will contribute to improve the photocatalytic activity of carbon dioxide reduction. Here we report that the n→π* transition in g-C3N4 is activated by the cooperation of steam etching and alkali treatment. The CO and CH4 evolution yields of the NaOH/Vc‐CN sample are 4.3 and 16 times higher than those of original g-C3N4, respectively. The planar asymmetry structure of heptazine was fabricated due to the hydroxyl groups reacting with terminal N-H content produced by the construction of carbon vacancy and the Na+ ions insert into the interlayer. Therefore, n→π* electronic transition in g-C3N4 was awakened, extending the optical absorption range with light wavelengths longer than 470 nm. At the same time, the ability of CO2 chemisorption and activation was improved due to the NaOH modification. Therefore, the extended visible light absorption, the improved crystallinity and the increased active sites are beneficial to optimizing the utilization efficiency of photogenerated carriers and enhancing photocatalytic activity.

Porous and Few-Layer Carbon Nitride Nanosheets via Surface Steam Etching for Enhanced Photodegradation Activity

Porous and Few-Layer Carbon Nitride Nanosheets via Surface Steam Etching for Enhanced Photodegradation Activity

Preparation of Hierarchically Porous Metal-Organic Frameworks via Slow Chemical Vapor Etching for CO2 Cycloaddition.

Hierarchically porous metal-organic frameworks (HP-MOFs) are a class of promising functional material with micropores, mesopores, and/or macropores, which can address the issue of slow mass transfer and less exposed active sites for primitive microporous MOFs. Despite many attempts that have been achieved through a variety of techniques to date, there is still a myriad of spaces that urgently need to be exploited. In this work, we report the novel synthesis of HP-MOFs via slow chemical steam etching. The preparation process can be subtly achieved using water vapor as an etchant; meanwhile, the addition of ethanol into the vapor atmosphere is carried out because it can stabilize the MOF framework well with its hydrophobic alkane tails, thereby slowing the etching rate toward MOFs, successfully realizing the controllable etching manner of MOF components. Furthermore, the joint influence of the water content and etching temperature on the MOF backbone structure etched has thus been investigated in detail. Impressively, we can harvest desired HP-MOFs with the retained crystalline structure at a water content of 50% and an etching temperature of 120 °C. The resulting HK-120/50 product etched exhibits excellent catalytic activity and stability in [2 + 3] cycloaddition of CO2 than pristine MOF, which can be attributed to the more exposure of active sites and the acceleration of mass transportation across the entire MOF skeleton. Noteworthy, the strategy proposed in this study may be extended to other HP-MOF construction systems due to the lability of most MOFs toward the chemical water vapor.

Facile Steam-Etching Approach to Increase the Active Site Density of an Ordered Porous Fe–N–C Catalyst to Boost Oxygen Reduction Reaction

Developing facile and effective strategies to improve the active site density of transition-metal and nitrogen codoped carbon (M–N–C) catalysts for oxygen reduction reaction (ORR) remains a challenge. Herein, we propose ordered templates and a steam-etching synergetic approach to increase the active site density of Fe–N–C catalysts with interconnected ordered porous structures. The steam etching corrodes inactive amorphous carbon while active sites are well preserved. X-ray absorption fine structure and fitting result reveal the uniform distribution of atomically dispersed Fe–N4 active sites. The steam etching dramatically increases the active site density by 4.6 times, which is verified by the NO adsorption–reduction experiments. As a result, the ORR mass activity of the ordered macroporous Fe–N–C catalyst treated by steam etching at 800 °C (OM–Fe–N–C–steam–800) is 1.8 times higher than that of Ar-protected one. The proton-exchange membrane fuel cell employing the OM–Fe–N–C–steam–800 catalyst delivered an enhanced peak power density of 0.78 W cm–2 compared to that of Ar-protected counterpart (0.63 W cm–2).

Nitrogen-doped porous graphene/MnO2 composite as sulfur hosts for lithium-sulfur batteries

Lithium‑sulfur batteries have been considered as a promising chemical power source owing to the low costs, high energy densities, and nontoxicity. However, sulfur-based cathodes have several shortcomings, including poor electrochemical activity and fast capacity attenuation. In this work, manganese dioxide (MnO2) and nitrogen-doped porous graphene (pNGA) were introduced to the sulfur cathode to improve its electrochemical activity and prevent the rapid decay of capacity. The pNGA/MnO2 composite with a three-dimensional architecture was fabricated by steam etching of nitrogen-doped graphene aerogel and chemical deposition of MnO2 nanoparticles. In this structure, the nitrogen-doped porous graphene acts as a skeleton to form a conductive network. The MnO2 particles anchored on graphene sheets have a good interaction with polysulfides. The as-prepared pNGA/MnO2@S cathode shows an excellent comprehensive electrochemical performance. The calculated specific capacities are 1144, 1007, 920, 852, and 762 mA h g−1 at 0.1C, 0.2C, 0.5C, 1C, and 2C respectively. In addition, the pNGA/MnO2@S cathode displays a good cycling performance: the specific capacity maintains at 887 mA h g−1 at 0.5C over 100 cycles.

Air‐Steam Etched Construction of Hierarchically Porous Metal‐Organic Frameworks

Main observation and conclusionThe introduction of mesoporosity into the microporous metal‐organic frameworks (MOFs) is expected to expand their applications. Herein, we report a green and facile method to obtain hierarchically porous MOF structures by using an air‐steam etching process. By virtue of the protonation reaction between the imidazole moiety and water vapor, the protonated imidazole related linkers leave the framework, resulting in the formation of mesopores in the zeolitic imidazolate frameworks (ZIFs), as exemplified by ZIF‐8. Given the mild etching process, the materials’ structural integrity and crystallinity are well maintained. Accordingly, the hierarchical porous ZIF‐8 exhibited enhanced performance in the dye removal as well as CO2 cycloaddition reaction with epichlorohydrin in comparison with microporous ZIF‐8, owing to the accelerated mass transfer arising from mesoporous structures. Remarkably, the proposed steam etching approach is generally applicable, which can be readily extended to other ZIFs, such as ZIF‐14, ZIF‐69, and ZIF‐71, thus representing a powerful strategy to construct hierarchically porous MOF materials.

Rationally designed N, P Co-doped porous film via steam etching as self-supported binder-free anode for high-performance lithium-ion battery

The rational and oriented design of a proper electrode structure is crucial for the realization of desirable properties. Herein, we report the design and synthesis of freestanding hybrid films that feature desirable one-dimensional (1D) structure and favorable electrochemical properties for Li-ions batteries (LIBs). Briefly, by offering a water-steam originated from the real-time thermal decomposition reactions during the carbonization to etch the nanofibers in-situ, the hierarchical porosity and N, P co-doping are simultaneously introduced into carbon nanofibers skeleton. The synergistic effect of these two defects engineering expose more active sites, which can provide a substantial pseudocapacitance to boost the lithium storage performance. In addition, the in-situ coating structure formed by electrospinning process is also helpful to alleviate the volume expansion of Fe3O4. Therefore, the prepared hybrid films display the excellent results by delivering an areal capacity of 1.0 mA h cm−2 at 2 mA cm−2 after 200 cycles in the half-cell. Meanwhile, a practical coin-type full-cell also delivers an areal capacity of 0.62 mA h cm−2 at 0.5 mA cm−2 after 100 cycles. This work paves the way to constructing rational electrode materials with controllable defect engineering for energy-related applications.

Steam Selective Etching: A Strategy to Effectively Enhance the Flexibility and Suppress the Volume Change of Carbonized Paper-Supported Electrodes.

Paper-supported electrodes with high flexibility have attracted much attention in flexible Li-ion batteries. However, they are restricted by the heavy inactive paper substrate and large volume change during the lithiation-delithiation process, which will lead to low capacity and poor rate capability and cyclability. Converting the paper substrate to carbon fiber by carbonization can substantially eliminate the "dead mass", but it becomes very brittle. This study reports a water-steam selective etching strategy that successfully addresses these problems. With the help of steam etching, pores are created, and transition-metal oxides are embedded into the fiber. These effectively accommodate the volume change and enhances the kinetics of ion and electron transport. The pores release the mechanical stress from bending, ensuring the sufficient bendability of carbonized paper. Benefiting from these merits, the steam-etched samples show high flexibility and possess outstanding electrochemical performance, including ultra-high capacity and superior cycling stability with capacity retention over 100% after 1500 cycles at 2 A g-1. Furthermore, a flexible Li-ion full battery using the steam-etched Fe2O3@CNF anode and LiFePO4/steam-etched CNF cathode delivers a high capacity of 623 mAh g-1 at 100 mA g-1 and stable electrochemical performances under the bent state, holding great promise for next-generation wearable devices.

Controllable etching of MoS2 basal planes for enhanced hydrogen evolution through the formation of active edge sites

The catalytic activity of molybdenum disulfide (MoS2) is associated with active sites located along the edges, whereas the MoS2 basal plane is regarded to be inert. However, it is a great challenge to develop a rational way for producing active edges efficiently. Herein, we report a novel, cost-effective top-down process in which we can create a high density of active edge sites on MoS2 basal plane by selective steam etching. The results show that the etched structure is strongly sensitive to the temperature, which creates 1D nano-channels, 2D in-plane triangular pits and 3D vertical hexagonal cavities on the MoS2 basal planes by elevating the temperature. The edge configuration is revealed to exhibit a distinct crystallographic orientation. Furthermore, we evaluate the corresponding enhanced electrocatalytic activity for the hydrogen evolution reaction (HER) by measurements of the single etched MoS2 samples in an electrochemical microcell, where the Tafel slope decrease by 49%, confirming the increased the density of active sites. In addition, the method is not limited to 2D materials in a flat geometry alone, but is also demonstrated on 0D MoS2 particles by in-situ transmission electron microscopy. The steam etching reported here offers an alternative avenue to engineer the surface structures of MoS2 facilitating the electrocatalytic applications of MoS2 for hydrogen production.

Fast and controlled fabrication of porous graphene oxide: application of AFM tapping for mechano-chemistry

This paper describes a novel method to fabricate porous graphene oxide (PGO) from GO by exposure to oxygen plasma. Compared to other methods to fabricate PGO described so far, e.g. the thermal and steam etching methods, oxygen plasma etching method is much faster. We studied the development of the porosity with exposure time using atomic force microscopy (AFM). It was found that the development of PGO upon oxygen-plasma exposure can be controlled by tapping mode AFM scanning using a Si tip. AFM tapping stalls the growth of pores upon further plasma exposure at a level that coincides with the fraction of sp2 carbons in the GO starting material. We suggest that AFM tapping procedure changes the bond structure of the intermediate PGO structure, and these stabilized PGO structures cannot be further etched by oxygen plasma. This constitutes the first report of tapping AFM as a tool for local mechano-chemistry.

One-step oxidation preparation of unfolded and good soluble graphene nanoribbons by longitudinal unzipping of carbon nanotubes

A simple one-step method to prepare graphene nanoribbon (GNR) is reported in this paper. Compared with water steam etching, the oxidation and co-etching of dilute sulfuric acid can result in the more complete longitudinal unzipping of carbon nanotube, although there is no other strong oxidant. As-prepared GNRs are more flat and have more oxygenated functional groups along the edge. Moreover, they can steadily disperse in a water system. These make them suitable as a carrier for supporting palladium (Pd) nanoparticles. The Pd/GNR composite exhibits a superior electrocatalytic activity for ethanol oxidation.

Enhanced gas sensing by 3D water steamed graphene hydrogel

The interaction area and defect sites play important roles in determining the gas sensing properties of graphene. In this work, steam etching is exploited to fragment large graphene sheets on 3D graphene hydrogel (GH) to small pieces with increased defect sites. The obtained steamed graphene hydrogel (S-GH) displays three and ten times higher responses to NO2 and NH3, respectively, compared with its unmodified GH counterpart before steaming treatment. Furthermore, the S-GH-based NO2 sensor exhibits a low limit of detection (LOD) of 57ppb, which is much lower than that of a GH-based NO2 sensor (178ppb). This work demonstrates a facile hydrothermal steaming method to enhance the performance of graphene-based gas sensor by combining 3D structural design and defect modulation of graphene materials.

Selective Etching of Nitrogen‐Doped Carbon by Steam for Enhanced Electrochemical CO2 Reduction

AbstractNitrogen‐doped carbon structures have recently been demonstrated as a promising candidate for electrocatalytic CO2 reduction, while in the meantime the pyridinic and graphitic nitrogen atoms also present high activities for electroreduction of water. Here, an etching strategy that uses hot water steam to preferentially bind to pyridinic and graphitic nitrogen atoms and subsequently etch them in carbon frameworks is reported. As a result, pyrrolic nitrogen atoms with low water affinity are retained after the steam etching, with a much increased level of among all nitrogen species from 22.1 to 55.9%. The steam‐etched nitrogen‐doped carbon catalyst enables excellent electrocatalytic CO2 reduction performance but low hydrogen evolution reaction activity, suggesting a new approach for tuning electrocatalyst activity.

Porous reduced graphene oxide sheet wrapped silicon composite fabricated by steam etching for lithium-ion battery application

A novel of Si/porous reduced graphene oxide (rGO) composite is fabricated by steam etching of Si/rGO aerogel. The rGO sheets with nano-holes build a unique three-dimensional porous network and can encapsulate the Si nanoparticles. The porous structure of Si/rGO composite can reduce the transfer distance of Li ions and restrain the aggregation and destruction of Si particles. The in-situ transmission electron microscopy (TEM) observation demonstrates that the porous rGO sheets help the entire electrode to maintain highly conductive and facilitate the lithiation of Si nanoparticles. The composite electrode presents high specific capacity and good cycling stability (1004 mAh g−1 at 50 mA g−1 up to 100 cycles).

Development of UV-assisted ozone steam etching and investigation of its usability for SU-8 removal

We have developed ultraviolet (UV)-assisted ozone steam etching as a novel SU-8 removal technology. This technology is expected to remove SU-8 at a higher etch rate compared to a conventional wafer cleaning process based on ozone water immersion. An etching apparatus prototyped in this study achieves an etch rate greater than 100 nm min−1 by intermittently rotating an SU-8-coated substrate on a spinner for spinning out the etching products under UV irradiation. Its complete removability is also verified by analyzing the surfaces of etched samples via scanning electron microscopy and x-ray photoelectron spectroscopy. In a micromolding process, the non-swelling removability is successfully demonstrated by removing a SU-8 sacrificial mold and leaving electroplated metal mushroom microstructures without destruction. Such a structure cannot be released by conventional resist strippers, because it is lifted up due to resist swelling. We believe this etching technology becomes a viable option to remove chemically-stable polymers such as SU-8 in microelectromechanical systems.

Clean unzipping by steam etching to synthesize graphene nanoribbons

A simple, clean method of steam etching carbon nanotubes is reported for the synthesis of graphene nanoribbons. The chemical mechanism of carbon nanotube unzipping is also discussed. As an anode catalyst, as-prepared graphene nanoribbons combined with Pt exhibit quite a high catalytic activity for ethanol electro-oxidation in fuel cells.

Nanostructured Few-Layer Graphene with Superior Optical Limiting Properties Fabricated by a Catalytic Steam Etching Process

Tailoring the morphology and structure of graphene can result in novel properties for advanced applications. Here, we demonstrate the fabrication of nanostructured few-layer graphene through a mild etching process via catalytic steam gasification of carbon by Fe nanoparticles (NPs). Controlling the reaction temperature, steam concentration, and the loading density of the NPs enables the fine-tuning of the etching level of graphene. Well-defined nanotrenches with a width of less than 25 nm were formed by channeling of the catalytic NPs. Etching caves and quasi-semicircular etched edges were observed as well. The nonlinear optical properties of the resulting nanostructured graphene were studied under a 532 nm nanosecond pulse laser through an open-aperture apparatus. At the same level of the linear extinction coefficient, it exhibits superior optical limiting performance in comparison with pristine graphene and C60, showing a large potential in nanophotonic devices. This enhancement is ascribed to the defec...

Dual Roles of Steam in the Dry Gel Synthesis of Mesoporous ZSM-5

Mesoporous HZSM-5 was synthesized with a SiO2/Al2O3 ratio of 100 using a steam-assisted crystallization method. The resulting material was characterized by N2 physisorption, powder X-ray diffraction, focused ion beam-scanning electron microscopy, energy dispersive X-ray analysis, X-ray photoelectron spectrometer (XPS) depth profiling, NH3-temperature programmed desorption (TPD), pyridine-TPD, and collidine-TPD. Crystallization time was varied to determine the effect on material physical properties, total acidity, accessible acid sites, and aluminum distribution within the zeolite particles. The Barrett, Joyner, and Halenda mesopore volumes, Brunauer, Emmett, and Teller surface areas, and total pore volumes decreased with longer crystallization times, while the mesopore sizes increased with time. At prolonged crystallization time, the material underwent dealumination via steam etching followed by reforming of material, thus blocking accessibility to the mesopores. The total acidity was optimized at 18 h of crystallization time. In addition to total acidity, the ratio of external and mesoporous acid sites to total acid sites decreased with increased crystallization time. Although a relatively uniform distribution of Al was observed across the center of the 18 h particles, XPS depth profiling showed an enrichment of aluminum on the outer shell of all HZSM-5 particles (“aluminum zoning”) with this steam-assisted crystallization method. Thus, crystallization time should be strongly monitored when using this dry-gel synthesis method with soft templates.

Porous B-doped graphene inspired by Fried-Ice for supercapacitors and metal-free catalysts

An easy and efficient approach inspired by the ‘Fried-Ice’ concept has been developed for the preparation of boron-doped graphene (B-G) with porous morphologies. The morphology, boron content, and specific surface area (SSA) of these porous graphenes could be well tuned by controlling the reaction temperature. Because of the steam etching effect, the as-prepared porous B-Gs have a high SSA of up to 622 m2 g−1. The porous B-doped graphene electrodes show promising performance in supercapacitors with a gravimetric capacitance of up to 281 F g−1. As metal-free electrocatalysts for the oxygen reduction reaction in alkaline medium, these electrodes facilitate the four-electron transfer process with high performances in limited current density and high onset potential (−0.12 V vs. SCE) that is close to that of the commercial Pt/C (40 wt%) and excellent tolerance to methanol and carbon monoxide.